Create redirect for Rguide.xml to Rguide.md

#459 replaced the R style guide and removed Rguide.xml, but we need this file to prevent broken links.

Rguide.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head>
+    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
+    <meta http-equiv="refresh" content="0; URL=Rguide.md"/>
+    <link rel="stylesheet" type="text/css" href="styleguide.css"/>
+    <title>Google's R Style Guide</title>
+  </head>
+  <body>
+    <h1>Google's R Style Guide</h1>
+    <p>
+      The style guide has moved to <a href="Rguide.md">Rguide.md</a>
+    </p>
+  </body>
+</html>